How perfect a Modigliani nude can be. Her skin is melon-coloured and her hair tumbles towards a waist of Posh Spice circumference. Unlike portraits by Lucian Freud or Francis Bacon, all flab and anguish, this woman belongs on a continuum of female beauty that began with Aphrodite. She is erotic, straightforward, tranquil. Or so it seems.

Today, visitors to the Royal Academy will be queuing to see her and her sisters. Modigliani and his Models is a diverse collection, but its highlights are the nudes he painted between 1917 and 1919. As ever, critics are divided. Some think him the king of monotony; others marvel at the tenderness and sensuality his subjects convey.

It is well known that 'Modi', a roisterous drunk, was never Mr Work-Life Balance. But there is little hint, among the praise or damnation, of how he really behaved towards his women. Amedeo Modigliani was a sexual predator whose models were procured by his art dealer. He boasted that 'to paint a woman is to possess her' and may have slept with all of them. But Modi's nudes did not belong to him for long. The sitting over, they were ejected to the sleazy bars they came from. They were the lucky ones.

His live-in muses, whose portraits are also on show, endured terrible violence. He slashed one lover's face with glass and hurled another, Beatrice Hastings, through a closed window. When she was carried inside, weeping and drenched in blood, Modi repeated: 'Non mea culpa' over and over. Many pundits seem almost as exculpatory.

One describes a canvas of Jeanne Hebuterne, his last partner, as resembling 'a Madonna, bringing serenity and warmth to the troubles of the world'. Quite what equipped Jeanne for this role is unclear. She was 19 when they began an affair in which he terrorised and reviled her. An onlooker who watched him throw her against railings and drag her along by her hair described him as 'like a madman, crazy with savage hatred'. The scene, according to Modi's biographer, Jeffrey Meyers, was 'a bit of street theatre for the delectation of his friends'.

Sent to register the birth of his daughter, Modi, who disowned a previous infant son, got drunk and never reached the town hall. Five months later, Jeanne, suffering postnatal depression, was pregnant again. Though she nursed Modigliani devotedly when he fell ill with tubercular meningitis, there is implicit criticism from some that she never called a doctor or tidied the alcohol bottles and sardine cans from the deathbed. A few days after her lover's demise, she killed herself and her unborn child by leaping from a window.

Artists have always had a licence to behave in ways that would not be tolerated in others. If only John Prescott had taken up pointillism instead of politics, he would not be in the mess he is today.

Nor do the lives of Modigliani's women detract from his brilliance. Indeed, nastiness often polishes great talent. Sickert might never have painted his waifs hunched on lumpy mattresses but for cruising the Camden underworld so keenly that some thought he might be Jack the Ripper.

The critic's role is not a moral one. The world has shifted gear since Kaiser Wilhelm ordered Edward Munch to lighten up, and since the French police shut Modi's 1920 exhibition because of the display of pubic hair. Censorship, though, is quite different from providing a backstory.

Knowing the depths of Modi's depravity is essential to understanding him and his women. Any World Cup WAG might think that the sweeter nudes, featuring Botox brows and bikini waxes, could pass for footballers' wives if only they lost six inches from their hips. Other pictures are more disturbing. Some models, with their truncated limbs and extruded torsos, look butchered into desirability. Some wear wanton leers. Some have blanked-out eyes, which supposedly depict depth of feeling, but which look to me like the mutilation inflicted by someone who doesn't want to meet a victim's stare. These are portraits by a man who despises women and very probably hates them.

Modi was tormented, too. The National Gallery's Rebels and Martyrs show mocks artists feigning to be tortured garret-dwellers, but he was the real deal. Untroubled by drawing-room etiquette, he racketed towards the pantheon of James Dean, Jeff Buckley, Kurt Cobain and all who live furiously and die young. Such people often wreak collateral damage. Why, after all these years, should the story of Modi's women still matter?

In part, because art is not uncoupled from society. Inner-city brawls are routinely described as Hogarthian, after the 18th-century chronicler of binge Britain. Picasso's Guernica, far more than the record of a bombed Basque town, is the epitaph to the Spanish Civil War and the enduring warning against fascism. True, Modigliani was no causist. He did not set out to be Goya, depicting the hell of war, or Stanley Spencer portraying heaven in Cookham Dean.

Even so, the life behind the portraits has a message for an era when women are objectified or held in scorn. Take the vilifying of Heather Mills McCartney, or the alleged assault in which a female contestant on Australian Big Brother was held down by one male housemate while another rubbed his crotch in her face. Meanwhile, post-feminism supposedly swings back to a pre-feminist era. Little girls are seduced by celebrity, and women sign up to a hyper-sexualised world in which Jordan is queen, glamour modelling is hot and a WI branch visits Spearmint Rhino to see how lap dancing is done.

Or so the theory goes. I don't believe that women have taken over male chauvinism, any more than I think that Modigliani's women, with their empty eyes and Barbie chests, were the first prophets of raunch culture. More plausibly, these girls, nameless and soulless, were pioneers of a celebrity universe. Plucked from obscurity and quickly cast aside, they were part of a sorority of victimhood that is dying out much more slowly than it should.

That is not a denial of Modigliani's talent. I love his less damaged-looking nudes, with their salon suntans and their ability to laugh at their creator. They look like women dreaming of hen nights or trips to Topshop, rather than sex with an absinthe-crazed monster.

But it's a pity that the master of beauty has been welcomed to Britain with so little reference to the ugly side of his work. Modigliani's models are a reminder, almost a century after he painted and discarded them, that women cast into the public eye must often pay too high a price.